Conventional SCR systems include SCR catalysts configured to effectively reduce NOx from exhaust gas streams during operation in normal-to-high exhaust temperature ranges (e.g., between about 250° C. and about 450° C.). However, during cold starts of an engine when exhaust gas temperatures may fall within low exhaust temperature ranges (e.g., between about 60° C. and about 250° C.), conventional SCR catalysts fail to effectively reduce NOx from exhaust gas streams. This is because at low temperature ranges, the engine typically does not generate enough NO2 (and oxidation catalysts do not convert enough NO to NO2) for effective NOx reduction, and it is difficult to provide enough ammonia for effective NOx reduction. To convert more NO to NO2 in low exhaust temperature operating ranges, an oxidation catalyst may be infused with large quantities of platinum. Such oxidation catalysts can be expensive and may introduce additional drawbacks.
Additionally, for effective NOx reduction in low exhaust temperature operation of an engine, conventional SCR systems require adjustment or retuning of the engine operating conditions to artificially increase the exhaust temperature and amount of NO2. Reconfiguring the operation of the engine to increase exhaust temperature and NO2 generation can result in excess exhaust pollutants (e.g., NOx and particulate matter) and fuel consumption, as well as lower engine durability.
Other approaches to reducing NOx in low exhaust temperature ranges may involve using different materials in the SCR catalysts. However, such approaches do not provide for sufficient NOx reduction at lower temperatures (e.g., between about 60° C. and about 250° C.). Additionally, known SCR catalysts using different materials tend to only trap NOx in ammonia nitrate, which can clog the SCR catalyst, rather than reducing the NOx to nitrogen and water.